This invention relates generally to D.C. brushless motors, and more particularly, to the structure of such a motor in which a drive circuit is installed.
D.C. brushless motors are widely used in various fields because of high durability and reliability. For instance, D.C. brushless motors are often used as small electric fans or blowers installed in motor vehicles. In such conventional D.C. brushless motors, a printed circuit board carrying drive coils, a position sensor and other circuit elements are mounted on a stator. Japanese Utility Model Provisional Publication No. 58-46283 discloses such a motor where a yoke used as a magnetic path is necessarily positioned at a back side of a disc-like printed circuit board, which back side is opposite to a front side facing a rotor of the motor. Furthermore, since pins or legs of various circuit elements attached to the printed circuit board protrude beyond the surface of the back side of the the circuit board, the yoke has to be provided at a small portion of the printed circuit board while a drive coil is mounted on the front surface because the pins or legs of the circuit elements are obstacles to the attachment of the yoke. As a result, an area of the yoke facing the rotor corresponds to a relatively small area of the rotor, and therefore reluctance of a magnetic circuit formed by the rotor and the stator, including the yoke, changes during one revolution of the rotor. Accordingly, the attractive force between magnets of the rotor and the yoke changes during one revolution to cause the occurrence of undesirable vibrations or cogging. Therefore, the advantage of coreless structure of such a D.C. brushless motor has hitherto been reduced to a large extent.
Moreover, since the circuit elements are exposed according to the structure disclosed in the above-mentioned prior art publication, it is necessary to provide a partition or to apply a sealing agent to the electrical conducting portions of the circuit elements so as to prevent the drive circuit from becoming dusty. This is disclosed in another Japanese Utility Model Provisional Publication No. 55-4991.